1. Field of the Invention
The invention is related to the packaging and encapsulation of semiconductor devices, including electronic devices, optoelectronic devices, microelectromechanical system (MEMS) devices, and high-frequency devices.
2. Discussion of the Background
In order to protect semiconductor chip devices from environmental hazards, device manufacturers have developed a variety of ways for hermetically encapsulating integrated circuit or discrete devices. Many of these techniques rely on adhesive seals or low-temperature solder for low temperature sealing of a cover to a printed wiring board of a substrate including integrated circuits or discrete devices, respectively.
Techniques, such as those disclosed by Rogers et al (U.S. Pat. No. 5,821,692), the entire contents of which are incorporated herein by reference, utilize peripheral seals around a device to provide one level of protection and then encapsulate the device in a fluorinated carbon liquid to add further protection from the environment. Techniques, such as those disclosed by Jacobs (U.S. Pat. No. 6,071,761), the entire contents of which are incorporated herein by reference, encapsulate devices in a polymer resistant to deterioration and resistant to ambient moisture. While effective in sealing the devices from the outside environment, the introduction of foreign materials into direct contact with the devices can impede operation of the devices and over time can contribute to the contamination and failure of the devices. Broom (U.S. Pat. No. 5,516,727), the entire contents of which are incorporated herein by reference, discloses contamination and failure of light-emitting diodes encapsulated in a resin.
A number of alternatives to resin encapsulation of semiconductor devices have been developed for providing sealed hermetic protection of the devices without direct contact between the resin and the encapsulated devices. For example, Bernstein (U.S. Pat. No. 5,501,003), the entire contents of which are incorporated herein by reference, discloses a process in which a non-conductive frame is attached to a conductive lead frame (i.e. a printed wiring board) containing connections to an assembled electronic package. The non-conductive frame is attached and sealed to the conductive lead frame using an ultraviolet curable epoxy applied around a periphery of the conductive lead frame. The electronic package is thus encapsulated having an interior air gap with no interior resin in contact with the encapsulated devices. Similarly, Suzuki et al (U.S. Pat. No. 5,866,942), the entire contents of which are incorporated herein by reference, disclose a metal base package for a semiconductor device in which a cap is adhesively bonded to either a laminated metal base or a metal foil pattern on an large scale integrated (LSI) chip.
The hermetic encapsulation of theses devices depend on the integrity and longevity of the adhesive and the adhesive bond. The adhesive bond is frequently made between dissimilar materials having dissimilar thermomechanical properties from each other and from the adhesive used. As a consequence, the adhesive bond is subject to failure with time and is especially prone to failure during heat cycles.
With the recent interest in micro-electrical mechanical systems (MEMS) devices has come the need for more critical device encapsulation techniques. Lin et al (U.S. Pat. No. 6,232,150), the entire contents of which are incorporated herein by reference, disclose unique problems associated with MEMS devices and disclose the need for reliable encapsulation of MEMS devices which does not contaminate or impede the operation of miniaturized mechanical MEMS devices. For example, MEMS devices require encapsulation without contact or contamination to the enclosed accelerometers, pressure transducers, gyroscopes, and micro-resonators. Furthermore, this encapsulation may require the enclosure of getters or other chemistry, designed to improve the device reliability, for example stiction reduction, without adversely affecting the effectiveness of this chemistry. Lin et al describe prior art encapsulation techniques for MEMS devices and the shortcomings of those approaches. In particular, Lin et al disclose the incompatibility of elevated temperature global heating for the encapsulation of MEMS devices. To alleviate global heating, Lin et al disclose the fabrication of microheaters which locally heat a bonding surface of a cover cap above a MEMS device, thereby sealing the cover cap to the body of the MEMS devices. The complexity of the patterning and the operation of the micro-heaters add cost and are deterrents to the acceptance and utilization of microheaters.